The present invention relates to the processing of integrated circuit wafers and equipment useful for such processing. More particularly, the present invention relates to processes and equipment for coating integrated circuit wafers which compensate for target erosion which occurs during consecutive wafer coating operations. The invention maintains a consistent distance between the wafer and the-target as consecutive wafers are coated from the same target and thus maintains a consistent and predictable coating thickness from wafer-to-wafer over the useful life of the target. The present invention is also directed to processing equipment which may be used to decrease the processing cycle time for each wafer processed with the processing equipment, and thus increase the productivity of the processing equipment.
To sputter coat a deposition layer onto a substrate such as a wafer in a sputter chamber, a robot arm places a wafer onto a receiver which is located in the chamber to position the wafer over a resistance-heated support member commonly referred to as a "heater". The heater moves through the receiver to position the wafer thereon (the first or lower position), and then moves upwardly into a second, upper position in close, preselected proximity with the target for deposition of the coating on the wafer. As the wafer is moved from the first position to the second position, it passes through a hanger on which a cover ring is suspended, and the wafer engages the ring and lifts the ring off the hanger.
To ensure proper coating characteristics, the top of the wafer should be located approximately 30 to 80 mm from the face of the target, and the edge of the wafer is masked by the ring to prevent passage of the reactive plasma in the chamber to the underside of the wafer. The actual target-to-wafer distance is selected to establish the required coating thickness and properties, and this established distance must be maintained with minimal deviation as each subsequent wafer is processed with the same target. However, as wafers are sequentially coated using the same target, the surface of the target is eroded, thereby causing the target to become physically smaller. Thus, the target-to-wafer distance increases unless the heater is moved further upward to compensate for target erosion.
To move the heater from the lower position at which it receives the wafer to the upper position at which the wafer is processed, a pedestal is connected to the underside of the heater and has a shaft extending out the bottom of the chamber where it is driven by a positioning device. A physical stop is provided in the positioning device to engage a finger on the pedestal to limit upward movement of the pedestal shaft into the chamber. By adjusting the location of this physical stop, the amount of travel of the pedestal from the first position to the second position before the physical stop engages the finger may be varied, and thus the travel of the heater inwardly of the chamber can be controlled. As the target erodes and the distance between the target and wafer increases, the location of the stop may be manually adjusted to increase the travel of the heater and thus reestablish the proper distance between the wafer and target. Eventually, the target will erode to a point where it must be replaced. In most instances, the target must be replaced after 20 mm of erosion has occurred.
To establish the initial target-to-wafer distance, and to re-set that distance by adjusting the stop, a number of wafers may be processed, and the thickness of the deposition layer created thereon may be measured, so that the stop may be finely adjusted to establish the target-to-wafer distance which provides the desired deposition thickness. To establish the desired deposition layer thickness, which results at the proper wafer-to-target distance, several wafers may be run, and the stop adjusted, until the deposition is optimized. In part as a result of the physical limitations of the mechanical stop, the acceptable tolerance on the desired wafer-to-target distance is one min. Thus, when the target erodes a sufficient amount to cause the deposition layer on the wafer to approach an unacceptable thickness, the stop is moved to compensate for that target erosion. It is contemplated to move the stop in increments of up to one mm to compensate for target erosion. To accomplish this adjustment, the processing in the chamber must be stopped, and a technician must make a manual adjustment to move the stop the desired distance.
It should be appreciated that a 1 mm adjustment of the target-to-wafer distance will result in an overall change of between 3% and 1.25% of the total wafer-to-target distance. This change could result in substantial differences in deposition thickness between the last wafer processed before adjustment and the first wafer processed after adjustment.
In one prior-art device, a worm drive is used to move a susceptor in a chemical-vapor deposition (CVD) chamber. However, no provision is necessary in that device to incrementally compensate for target erosion from wafer to wafer.
In another prior-art device, the positioning device moves the pedestal through a pneumatic piston actuator which is disposed parallel to the pedestal shaft. A control rod extends from the piston actuator and terminates in a crossbar which is also linked to the pedestal shaft. The pedestal shaft has two positions: the pedestal-retracted position, in which the piston control rod is fully extended to hold the pedestal down in the retracted position and thus the heater in the first position; and, the pedestal-extended position, wherein the piston control rod is retracted into the pneumatic piston to move the pedestal upwardly in the chamber until the stop engages a manually adjustable limit finger on the pedestal, and thus positions the heater in the second position. Again, no provision is made to incrementally compensate for target erosion from wafer to wafer.
This prior-art method and apparatus for controlling the wafer-to-target distance has several disadvantages. Adjustment of the wafer-to-target distance may only be accomplished when wafer processing is stopped, by adjusting the location of the stop to permit greater pedestal travel before the finger engages the stop. Further, even after the stop is adjusted, the effect of the adjustment must often be evaluated empirically by measuring deposit thickness, and at times further adjusted, before the wafer-to-target distance is properly established. As the wafer-to-target distance commonly must be adjusted after every 100 wafers, the adjusting process is time-consuming and expensive. Additionally, the movement of the heater and wafer from the first position to the second position must be accomplished at a relatively low rate of speed, to ensure that the engagement of the wafer against the cover ring does not cause the wafer to chip or crack. Because the pedestal speed of the prior-art devices is not adjustable while the heater and wafer thereon are in motion, the entire movement of the heater and wafer is governed by the maximum permissible engagement speed of the wafer against the cover ring. This results in substantial dead time during processing, while the pedestal slowly moves the heater up to engage the cover ring on the wafer and then position the wafer adjacent to the substrate for processing.
Wang et al. in U.S. Pat. No. 4,872,947 describe a CVD plasma reactor in which the wafer is supported on one electrode which can be variably spaced from the other electrode to better control the plasma.